Wool and wool-blend fabrics have been processed and treated for many years to improve and/or enhance a wide range of characteristics. For example, the pre-treatment of fabrics, such as wool, before printing is essential to achieve good colour yields, levelness and brightness. Similarly a range of processes and treatments have been proposed to reduce or eliminate pilling.
Traditionally, chlorination has been used and several variants of the chlorination process are still used almost exclusively to prepare wool fabrics for printing. Dichloroisocyanuric acid (DCCA) is the most common chlorination reagent currently in use, and can be applied by both batch (the most common) and continuous processes. The batch method involves chlorination with 3-4% DCCA on mass of fibre (omf), at pH 3.5-4.5 and a temperature of 20.degree.-40.degree. C. for about 1 hour, followed by an antichlor aftertreatment with sodium bisulphite and acetic acid. The continuous process involves padding DCCA (35-50 gl.sup.-1), followed by a dwell time of 2-5 minutes before rinsing and an antichlor treatment similar to the batch process. The alternative to DCCA is Kroy chlorination, originally introduced for treatment of wool tops, which uses a solution of chlorine gas in water in a continuous fabric treatment process. Chlorine reacts with water to give a mixture of hypochlorous and hydrochloric acids, which is sprayed directly onto the fabric with a wetting agent. The reaction is more rapid than DCCA, but a rinsing and antichlor treatment are still necessary. Processing speeds of 10-15 m min.sup.-1 at a chlorine dose rate of 4% omf are typical and give similar performance to fabrics treated with 4% DCCA.
Typical problems with fabric chlorination include: yellowing, achieving an even application, and fibre damage. It is also very often necessary to bleach chlorine-treated fabrics, usually with hydrogen peroxide, to remove yellowness before printing. However, it is the environmental pressure on processes involving chlorine, particularly when absorbable organohalogens (AOX) are present in the plant effluent, which is leading to the replacement of chlorination by alternative technologies.
Other methods used to treat fabrics prior to printing are not common. Two polymer treatment routes, one for top and one for fabric are currently known, they are:
1. Hercosett 125 (trade name)
This polymer is applied to wool top after a prechlorination stage. Fabrics produced from treated top have an increased affinity for anionic dyes. The further mechanical processing which occurs during gilling, spinning and weaving results in a level preparation. However the colour yields tend to be lower since less chlorine is used. Further, care must be taken in washing off since treated wool has a high affinity for loose anionic dyes.
2. Synthappret BAP (trade name)
This polymer may be applied to a fabric without the need for a prechlorination step. The treatment of fabrics with this polymer prior to printing provides the fabric with a high affinity for hydrophobic dyes. However, the lack of a chlorination step reduces the penetration of printing paste into the fibres, and control over the steaming conditions is critical. This method has been used to print wool/cotton blends, but not pure wool fabrics to date.
Other methods avoiding the use of chlorine have been developed but are not considered to be commercially viable despite their reduced environmental impact. To summarise, the only prior art methods widely used commercially for pretreating wool fabrics for printing involve chlorination, followed by rinsing and an antichlor treatment, which then may require a bleaching treatment to remove yellowness.
Pilling is a term used to describe the formation of small, tight balls of fibre on a fabric surface. Pilling is highly detrimental to garments, resulting in a worn and unkempt appearance, and is a particular problem for knitwear.
The pilling process is complex but can be described as four successive stages:
(i) Fuzz Formation. The mild rubbing action which occurs during wear teases some surface fibres from their parent yarns, resulting in a fuzzy surface. PA1 (ii) Fuzz Entanglement. Areas of the garment which are subjected to more frequent rubbing develop the higher fuzz densities. Fibres in such areas become entangled at some stage to form loose balls. PA1 (iii) Pill Formation and Growth. Continued rubbing on loose entanglements causes some to roll into tighter balls. These tight balls resist further rubbing forces, and some of the weaker fibres in the pills break. The stronger fibres remain intact and anchor the pills to the fabric surface. Pills grow as they pick up loose fibres from the fabric surface. PA1 (iv) Pill Wear-Off. The anchor fibres finally succumb to the steadily increasing forces acting on the pill and undergo fatigue failure. As each anchor fibre breaks, those remaining have to withstand larger forces and the rate of anchor failure thus accelerates. Pill removal occures when the rate of anchor fibre breakage exceeds the rate of pill growth. PA1 (i) exposing the fabric surface to UV radiation; and PA1 (ii) oxidative treatment of the fabric.
The nature of the fibres (origin, processing history, physical dimensions), the yarn (type, twist) and the fabric structure are all important factors in pilling. In wear there are other variables which can influence the rate of pilling. It is well known that some wearers produce more rapid and extensive pilling than others. Laundering can substantially alter pilling performance. Subjective differences between individuals also exist over how objectionable a given amount of pilling is.
Several chemical treatments are known to reduce pilling, although as yet no process can guarantee zero pilling in wear. For example, the oxidative chlorination processes commonly used for shrinkproofing have some beneficial effect. Chlorine/Hercosett and certain other polymer treatments which inhibit fibre migration by forming inter-fibre bonds, are also beneficial. More damaging dyeing conditions (i.e. long boiling times, high temperatures, extremes of pH) also tend to reduce pilling.
Similar to printing pretreatments there is currently a great deal of environmental pressure against the use of processes which use chlorine, particularly when adsorbable organohalogens (AOX) are produced in plant effluent. Hence it is likely that the partially-effective anti-pilling treatments and printing pre-treatments which involve chlorine compounds will be phased out within the next ten years or so.
Applicant has now surprisingly found that the combination of subjecting a fabric to UV radiation followed by oxidative bleaching provides a synergistic mechanism to effectively increase the ability of the fabric to give good colour yield when printed and reduce the likelihood of pilling.
Extensive investigations involving the use of either UV radiation or oxidative bleaching alone established that the single steps were ineffective in increasing colour yields or reducing pilling significantly. It was also established that the oxidative bleaching step must follow the irradiation, and cannot be applied first or during irradiation while wet. It was found that high, even colour yields, better than those produced by 4% DCCA, were achieved using the two-step procedure over a range of classes of dye.
Most research on the effects of UV on wool has been aimed at limiting the long-term negative effects such as photoyellowing, phototendering and the fading of dyed wool. Previous work on the positive application of UV radiation (.lambda.&lt;400 nm) in the treatment of wool fabrics appears to be limited to two commercial patents.
U.K. Patent 811702 describes the use of ultraviolet radiation for modifying the rate of dye uptake of wool fabrics. This increases the colour yields of exposed fabric, depending on the nature of the dye used. Use of suitable stencils during irradiation, followed by use of a dye resist agent to partially protect unirradiated areas of fabric during dyeing, can produce good tone-on-tone effects. This document also discloses that in the interest of shortening the period of irradiation it is advantageous to treat the fabric with an oxidising agent during UV exposure. However, this document does not describe or suggest the possible application of irradiation to fabric printing, or the method of oxidative bleaching of the fabric after subjecting the fabric to irradiation. In fact, the U.K. patent stresses the use of an oxidising agent during UV exposure to shorten the period of irradiation rather than as an essential, discrete step in a synergistic process to increase the affinity of the fabric to dyes.
Japanese Patent H4-41768 claims that UV exposure alone is an effective shrinkproofing treatment for wool fabrics. However the claimed large reductions in fabric area shrinkage have not been reproduced in our studies. This could be due to the nature of the wool fabric used by the Japanese workers, or because their felting procedure was less severe than ours.